Halibut fishing in Seward, AK

A writing project presentation

Leslie Gains-Germain

"We travel farther for smaller fish"

Questions of Interest

Data from Alaska Department of Fish and Game

Question 1: Is there evidence that the distance Seward charter boats travel to fish for bottomfish has changed over the years 1993-2014?

Logbook Data

Interview Data

Question 2: Is there evidence of a change in the mean length of halibut caught by Seward fishermen over the years 1994-2013?

Biological Data

Data Reference

ADF&G Saltwater Logbook Database [Intranet]. 2004–2013. Anchorage, AK: Alaska Department of Fish and Game, Division of Sport Fish (cited 4/2/2015). Custom query details available upon request from ADF&G, Division of Sport Fish, Research and Technical Services.

ADF&G non-confidential data. 1993-2014. Homer, AK: Alaska Department of Fish and Game, Division of Sport Fish. Personal communication, Scott Meyer.

ADF&G non-confidential data. 1994-2013. Homer, AK: Alaska Department of Fish and Game, Division of Sport Fish. Personal communication, Scott Meyer.

Logbook Data 2004-2013

Logbook: 2004-2013

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Interview Data 1993-2014

Two days a week randomly selected for interviews
Effort spread across the four harbor cleaning stations
Boat selection is pseudo-random

Did you catch bottom fish today?
If yes, record trip type
Identify the primary statistical area where the majority of bottomfish were caught.

abbreviation	description
H	halibut exclusively
B	any combination of bottomfish (halibut, rockfish, lingcod, Pacific cod, sharks)
B+S	any combination of bottomfish and salmon (e.g. halibut+coho)

Interview 1993-2014

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Comparison - Resurrection Bay

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Comparison - Cape Cleare

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Comparison - Patton Bay

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Fitted Probabilities and 95% Pointwise CIs

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The odds of fishing in Cape Cleare in \(2014\) are estimated to be \(2.43\) times the odds of fishing in Cape Cleare in \(1993\), with a \(95\%\) confidence interval from \(1.22\) to \(4.92\) times.

The odds of NOT fishing in Resurrection Bay in \(2014\) are estimated to be \(2.78\) times the odds of NOT fishing in Resurrection Bay in \(1993\), with a \(95\%\) confidence interval from \(1.32\) to \(5.82\) times.

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Question 2: Is there evidence of a change in the mean length of halibut caught by Seward fishermen over the years 1994-2013?

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Biological Data 1993-2013

Three days a week randomly selected for biological sampling
Boats selected (non-random) and entire catch is studied

port	sector	date	age	length	sex	statarea	length.in	area
Seward	Charter	5261994	11	120.0	F	495938	47.24	resbay
Seward	Charter	5261994	NA	128.0	F	475934	50.39	capecleare
Seward	Charter	5261994	13	85.0	M	475934	33.46	capecleare
Seward	Charter	5261994	13	85.5	M	475934	33.66	capecleare
Seward	Charter	5271994	11	126.5	F	485935	49.80	junken
Seward	Charter	5271994	11	95.8	F	475934	37.72	capecleare

Average fish lengths plotted over time

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Mixed Effects Model

Each area modeled separately

\(length_{i}=\beta_0+\beta_1year_i+\alpha_{i}+\epsilon_{i}\)

\(\epsilon_{i} \overset{iid}{\sim} N(0, \sigma^2)\) \(\alpha_{i} \overset{iid}{\sim} N(0, \sigma^2_{\alpha})\)
\(i \in (1994, 1995,...,2013)\)

In the waters around Cape Junken, there is strong evidence of a relationship between mean fish length and time over the years \(1994\) to \(2013\) (p-value\(<0.0001\) from t-stat\(=-5.370\) on \(18\) df).

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The mean fish length in Cape Junken in \(2013\) is estimated to be \(6.8\) inches shorter than the mean fish length in Cape Junken in \(1994\), with a \(95\%\) confidence interval from \(4.35\) to \(9.29\) inches shorter.

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Scope of Inference

Technically does not extend to all Seward boats/fish, BUT....

No causal

Questions?